A reduced oxygen atmosphere during melt processing turned out to be critical for the fabrication of NdBa2Cu3Oy (Nd123) superconductors possessing high superconducting transition temperature (Tc) with a sharp transition and large critical current density (Jc) at 77 K. In a dc magnetization measurement, Nd123 superconductors melt processed in flowing a mixture gas of 1% O2 in Ar exhibited the Tc of about 95 K and the transition width of 1.5 K with the applied field of 10 Oe. A four‐probe measurement showed the zero resistive transition Tc (R=0) of about 95 K. An anomalous peak effect in the magnetization hysteresis (M‐H) loops was commonly observed and lead to large magnetic Jc of 2×104 A/cm2 at 77 K and 2 T for the applied field H parallel to the c axis of a sample (H∥c). This achievement is attributable to a preferential formation of high Tc phase (x<0.1) among the Nd1+xBa2−xCu3Oy solid solutions in a reduced oxygen atmosphere.

Melt processing for obtaining NdBa2Cu3Oy superconductors with high Tc and large Jc

Unlike Y123 which forms only a stoichiometric compound, the light rare earth elements (LREs: La, Nd, Sm, Eu, Gd) form a solid solution . The presence of such solid solution caused a depression in the superconducting transition temperatures , particularly for La123, Nd123 and Sm123 when they are melt processed in air. Recently, we have found that the of these LRE123 superconductors can greatly be enhanced when they are melt processed in a reduced oxygen atmosphere. Furthermore, values of these superconductors were larger than that of a good quality Y123 superconductor in high magnetic fields at 77 K. In this article, on the basis of our study over the last several years, the melt processes for LRE - Ba - Cu - O are described, the microstructural and superconducting properties of the superconductors are reviewed and the flux pinning mechanism is also discussed.

https://iopscience.iop.org/article/10.1088/0953-2048/9/12/001/pdf

Melt-processed light rare earth element - Ba - Cu - O

The potential of high temperature superconductors to generate large magnetic fields and to carry current with low power dissipation at 77 K is particularly attractive for a variety of permanent magnet applications. As a result large grain bulk (RE)-Ba-Cu-O ((RE)BCO) materials have been developed by melt process techniques in an attempt to fabricate practical materials for use in high field devices. This review outlines the current state of the art in this field of processing, including seeding requirements for the controlled fabrication of these materials, the origin of striking growth features such as the formation of a facet plane around the seed, platelet boundaries and (RE)2BaCuO5 (RE-211) inclusions in the seeded melt grown microstructure. An observed variation in critical current density in large grain (RE)BCO samples is accounted for by Sm contamination of the material in the vicinity of the seed and with the development of a non-uniform growth morphology at ≈ 4 mm from the seed position. (RE)Ba2Cu3O7-gd (RE-123) dendrites are observed to form and broaden preferentially within the a/b plane of the lattice in this growth regime. Finally, trapped fields in excess of 3 T have been reported in irradiated U-doped YBCO and (RE)1+xBa2-xCu3Oy (RE = Sm, Nd) materials have been observed to carry transport current in fields of up to 10 T at 77 K. This underlines the potential of bulk (RE)BCO materials for practical permanent magnet type applications.

Processing and properties of large grain (RE)BCO

We have studied the superconducting properties of Nd(Ba1-xNdx)2Cu3O7-δ (Nd123, x≈0.1) single crystals grown by the traveling-solvent floating-zone method under 0.1% O2 in Ar atmosphere. An anomalous peak effect in the magnetization hysteresis (M-B) loop is observed in the Nd123 single crystals as well as in the Nd123 bulk crystals prepared by the oxygen-controlled-melt-growth (OCMG) method. The critical current density (JC) of the Nd123 single crystals is 70,600 A/cm2 in 1.0T at 77K for the applied field perpendicular to the ab-plane. The existence of Nd and Ba composition variation in the matrix of Nd123 crystals is observed by an analytical TEM equipped with a field-emission gun. The high Jc value of the Nd123 single crystals in the applied field is explained by the field-induced pinning centers caused by the Nd-Ba substitutions in the Nd123 matrix.

High critical‐current density of Nd(Ba,Nd)2Cu3O7−δ single crystals

Volume pinning forces, F p , are measured in a wide temperature (5 ≤ T ≤ 92 K) and field range (0 ≤ μ0 H a ≤ 9 T) on different NdBa2Cu3O7-δ samples. Above 60 K a good scaling of F p , versus the reduced field, h = H a /H irr can be established. The scaled pinning curves are compared to several theoretical predictions. Experimental evidence for strong pinning at extended (Vpin ~ ξ2 d) defects is given. These defects are ascribed to spatial composition fluctuations found in light rare earth superconductors, providing a scatter of the transition temperature, T c . Such a pinning echanism is especially important for applications of high-T c superconductors operating at 77 K.

Analysis of pinning in NdBa2Cu3O7-δ superconductors

REBa2Cu3Oy superconductors with rare earth (RE) ions with large radii (RE: La, Nd, Sm) exhibit relatively low T c due to the presence of RE-Ba solid solution. We have found that this solid solution can be suppressed if these superconductors are melt processed in a reduced oxygen atmosphere. We have also found that critical current densities of these superconductors are higher than those of melt processed YBa2Cu3Oy with fine Y2BaCuO5 inclusions in a high field region. The irreversibility line was also shifted toward the higher H-T region. We believe that flux pinning in these superconductors is ascribable to a finely distributed RE(Ba1-x , REx )2Cu3Oy phase in a good superconductive matrix.

https://iopscience.iop.org/article/10.1143/JJAP.33.L715/pdf

Jurgen Weltz

Papers

Flux pinning in melt-grown NdBa2Cu3Oy and SmBa2Cu3Oy superconductors